1. Field of the Invention
The present invention relates to an automatic transmission for an automobile and more particularly to a hydraulic control apparatus of an automatic transmission.
2. Prior Arts
Generally, in automatic transmissions for automobiles, gear ratios are automatically selected according to vehicle operating conditions such as engine load, vehicle speeds and the like and those gear ratios are obtained by selectively engaging friction elements such as clutches and brakes in automatic transmissions. In case of the automatic transmission employing a planetary gear for the transmission gear, the frictional engaging elements are composed of hydraulically controlled multiple disc clutches and hydraulic brakes.
The control of these friction elements is a key technology of the automatic transmission and many techniques have been proposed. Among them, Japanese Patent Application Laid-open (Unexamined Publication) No. Toku-Kai Hei. 7-269685, which has been proposed from the inventor of the present invention, discloses a so-called direct shift control method using pressure control valves provided in respective friction elements.
In the prior art, an electromagnetic type pressure control valve operated on forward running is provided on the friction elements, respectively and each of these pressure control valves performs a shift control based on an independent transient hydraulic characteristic so as to alleviate a shift shock and at the same time to shorten a shift time.
Further, Japanese Patent Application Laid-open No. Toku-Kai-Hei 7-269685 proposes a hydraulic control apparatus having two fail-safe valves for automatically avoiding such a hazardous condition as both low and reverse brake and overdrive clutch are simultaneously engaged when the gear is engaged in a first gear at "D" range.
However, the electromagnetic type pressure control valve is complicated in construction and therefore high in manufacturing cost. Furthermore, since one electromagnetic type pressure control valve must be provided for each of the frictional engaging elements, not only the total cost becomes high, but also the size of the apparatus increases.
Further, the fail-safe valves proposed in Toku-Kai-Hei 7-269685 makes the hydraulic control system more complicated and increases the size and cost of the apparatus.